Taking a cold cathode fluorescent lamp CCFL as a light source has the shortcomings as below. The CCFL emits light with wavelength 185 nm which causes a new absorptive spectrum band and the brightness of emitting light degrades with time; the recombination of Hg+ and the other ions at the tube wall disrupts the lattice structure of the phosphor layer exaggerating the brightness degradation of the emitting light. Moreover, for CCFL, the phosphor coating and the light emitting source have to be disposed in the same vacuum lamp tube, and therefore it is difficult to produce large-sized lamps and to provide a wavelength conversion structure with large emitting area only by CCFL. Another disadvantage is that the phosphor coating mentioned above is uneven in most cases. Therefore, the light emitted by the CCFL suffers the problem of non-uniformity.
Other light sources, including external electrode fluorescent lamps (EEFL), light emitting diodes (LED), carbon nanotubes (CNT), flat fluorescent lamps (FFL), and organic light emitting displays (OLED) are either immature for production (e.g., LED, CNT, OLED and FFL) or inapplicable for large-size applications. More importantly, being a flat light source, these existing light sources are expensive and complex.
The light sources mentioned above are line source or point source. If the distance between the light sources mentioned above is too far or the distance between the light source and a diffusion plate or a guiding plate is too narrow, a phenomenon called “mura” appears or becomes exaggerated. Mura effect is a problem that should be paid attention to while utilizing the traditional light sources listed above for producing a light source with uniformity. Mura effect here means the luminance difference of a lighting area. The phenomenon may be caused by reflection of incident light, interaction of light field, position of the light sources, for example, interference of light, luminance distribution, or both.
FIG. 1 discloses a flat light source 1 having a light emitting layer 11, a diffusion plate 12 for diffusing the incident light, lamps 13 and other optical structures 14. The flat light source 1 is designed for providing emitting light with lowered mura. Referring to FIG. 2, the non-uniformity ratio of the emitting light provided by the flat light source is lowered while the diffusion angle is lower than about 150 degree, but the emitting light provided by such a flat light source still suffers from undesired mura effect.
To provide a light source with less mura, the traditional way is to provide a diffusion plate or an optical structure with ink printed on the surface. Both solutions lower the luminance of emitting light, and the latter solution also requires aligning the position of ink with the light source with high precision.
Many inventions have been provided, too. Some inventions relate to improving the optical character of the diffusion plate. U.S. Pat. No. 7,290,921 provides a light guide plate with sub-scattering-dots for producing uniform emitting light. The technical feature of this invention is providing scattering-dots and sub-scattering-dots at a predetermined region on the bottom surface of said light guide; wherein at least one sub-scattering-dot is disposed around each scattering-dot and the sub-scattering-dots are smaller than the scattering-dots. U.S. Pat. No. 7,018,059 provides a direct type backlight module; wherein, a diffuser plate disposed on the reflector, and the lamps are disposed between the reflector and the diffuser plate. This invention provides a light-distributing device to inhibit mura effect. The light distributing device is a prism plate, a metallic adhesive layer with a plurality of holes or a light guide plate with various indexes of refraction thereon.
Some inventions provide improvement of the surface character. U.S. Pat. No. 7,172,331 provides a direct type backlight module having a holding structure for holding a reflector on a frame, instead of using an adhesive to prevent roughness and unevenness in the reflector's surface when the reflector is attached onto the surface of the frame.
In view of the above, no invention has been found relating to controlling the mura of a lighting area by manipulating the configuration of a wavelength converting structure. Therefore, a patterned wavelength converting structure is disclosed herein.